One of the most important characteristics required for steel materials used for structures such as ships, buildings, pressure containers, linepipes, etc, is HAZ toughness. Recently, heat-treating technologies, controlled rolling and machining heat-treating method have made a remarkable progress, and the improvement of low temperature toughness of the steel material itself has become easy. Because the welding HAZ is reheated to a high temperature, however, the fine structure of the steel material is completely lost, and its microscopic structure becomes extremely coarse to thereby invite drastic deterioration of HAZ toughness. Therefore, as means for refining the HAZ structure, (1) a technology for limiting coarsening of austenite grains by TiN and (2) a technology for forming intergranular ferrite by Ti oxides, have been examined and put into practical application. For example, CAMP-ISIJ Vol. 3 (1990) 808 describes the influences of N on intergranular ferrite transformation in Ti oxide type steels, and Vol. 79 (1993) No. 10 describes the effect of B on intergranular ferrite transformation in Ti-containing oxide steels. Nonetheless, the level of HAZ toughness produced by these technologies is not yet entirely satisfactory. From the aspect of execution of welding, therefore, a steel material which has higher strength and can be used at a low temperature and with a large heat input has been strongly desired.